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Brushless motors gain ground

Posted: 02 Oct 2006 ?? ?Print Version ?Bookmark and Share

Keywords:brushless motor? brush motor? MCU? microcontroller? Apex Microtechnology?

Cox: Much work remains to be done in the marketplace to provide designers with the integrated, brushless control circuitry options they now desperately seek.

"It's about time" would be an understatement regarding the steady, growing and newfound popularity of brushless motors. These lighter-weight wonders are steadfastly gaining ground on their century-old brethren, the brush motors.

Two years ago, brushless motors were more expensive than brush motors. But thanks to advances in design and materials, brushless motors have experienced a drop in pricing. Today, the cost differential between the two motor technologies can be just 10 percent. The most notable converts are designers working with industrial applications. This was traditionally considered hard-core brush territory because clean operating environments are not extremely important. But now, the brushless motor is finding a new fan base because of its declining cost barrier.

The most notable difference between the two motors is the absence of the familiar brush-commutator arrangement. For over a century, commutation for brush-type DC motors has been performed electromechanically via graphite brushes that contact a circular commutator mounted on the rotor.

Thanks to Hall sensors that feed back the rotor position, the control circuitry always knows the exact moment to commutate (sequence) the motor phases. Most brushless motor manufacturers supply motors with three Hall-effect position sensors. Because brushless motors lack brushes and thus this interface, they are cleaner and virtually maintenance-free. They exhibit lower acoustic noise and last longer.

So what's not to like about them? Despite all the excitement surrounding the brushless wonders of the motor world, work still has to be done to develop the aforementioned control electronics of the brushless motor's Hall sensors.

Motor drive boxes and drive cards that provide the MCU, the programmability and the driver in a single assembly are in steep demand. In essence, this integration scenarioeither in digital or analog modeperforms the necessary commutation in each motor scenario. Without this integration, the brushless motor is rendered idle.

PWM solution
When it comes to selecting the best driver, PWM ICs are becoming the preferred technology. Selecting the fittest is simply based on efficiency. The disadvantage of the linear circuit becomes clear in the midrange, when the output level is in the vicinity of 50 percent. At these levels, the resistance of the pass element is equal to the load resistance, which means that the heat generated in the amplifier is equal to the power delivered to the load. More simply stated, a linear control circuit exhibits a worst-case efficiency of 50 percent when driving resistive loads at midrange power levels.

Now, consider a PWM solution. In a PWM control system, an analog input level is converted into a variable-duty-cycle switch drive signal. The process of switching from one electrical state to another, which is the difference between on and off, is called modulation, hence the technique's name. Beginning at zero duty cyclei.e. off all the timethe duty cycle is often advanced as the motor begins to rotate until it is running at the speed and/or the torque required by the application. Again, what's not to like?

MCUs with special functions (routines) necessary for controlling brush motors and PWM IC drivers are beginning to emerge. But much work remains to be done in the marketplace to provide designers with the integrated, brushless control circuitry options they now desperately seek.

- David Cox
VP of engineering, Apex Microtechnology Corp.

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